Stripline design for the extraction kicker of Compact Linear Collider damping rings

Abstract

In the framework of the design study of future linear colliders, the Compact Linear Collider (CLIC) aims for electron-positron collisions with high luminosity at a nominal center-of-mass energy of 3 TeV. To achieve the luminosity requirements, predamping rings (PDRs) and damping rings (DRs) are required: they reduce the beam emittance before the beam is accelerated in the main linac. Several kicker systems are needed to inject and extract the beam from the PDRs and DRs. In order to achieve both low beam coupling impedance and reasonable broadband impedance matching to the electrical circuit, striplines have been chosen for the kicker elements. In this paper, we present the complete design of the striplines for the DR extraction kicker, since it is the most challenging from the field homogeneity point of view. The excellent field homogeneity required, as well as a good transmission of the high voltage pulse through the electrodes, has been achieved by choosing a novel electrode shape. With this new geometry, it has been possible to benefit from all the advantages that the most common shapes introduce separately. Furthermore, a detailed study of the different operating modes of a stripline kicker allowed the beam coupling impedance to be reduced at low frequencies: this cannot be achieved by tapering the electrodes. The optimum design of the striplines and their components has been based on studies of impedance matching, field homogeneity, power transmission, beam coupling impedance, and manufacturing tolerances. Finally, new ideas for further improvement of the performance of future striplines are reported.

Fields and characteristic impedances of dipole and quadrupole cylindrical stripline kickers

“…With either Z c,even or Z c,sum set equal to Z L = R 11 the even or sum mode is matched; the remaining equation(s) can be solved for the loads needed to match other modes. The result for n = 2 is identical to that presented in section VI E of reference [9]. The generalization to arbitrary n is straightforward.…”

“…A more complete discussion of the odd and even modes can be found for example in [13]. If the beam current is high enough for beam instabilities to become a concern, the characteristic impedance of the even mode should be optimized to prevent the field in the resonator formed by the plates from acting back on the circulating beam [9]. As is discussed later in Section 2.4, in general matching the geometric means of both modal impedances to that of the external lines may be the best compromise.…”

“…Appendix C shows that in general a load termination network can be devised to match all modes with any number of plates. For a dipole configuration, this requires an additional resistor between the two electrodes, a scheme that was also proposed in [9].…”

“…Detailed designs are usually done with electromagnetic codes which solve for the fields using a variety of numerical schemes, see e.g. [8,9]. In this paper, we focus instead on analytical methods to solve Laplace's equation.…”

The CLIC project

“…It is equipped with electrodes with a novel shape, called half-moon electrodes. The electrode support, feedthroughs and manufacturing tolerances have been optimised to match the impedance during operation and to minimise the field inhomogeneity [71,72]. A prototype of this kicker, shown in Figure 2.10a, has been manufactured in a collaboration between CERN, CIEMAT and IFIC in Spain [73].…”